final update, broadband connection now seems stable delivering a more or less consistant 48Mbps which would probably equate to the sync speeds of 51 that you are getting. Also I have not noticed any dropouts.
I currently regard this matter as closed but will contact you again should further problems occur. Thank you for all your help,
I'm seeing a redirect in the authentications logs from earlier.
Can you turn off the router for 30 minutes. This will allow the current session to close.
Turn on the router after this and it will reconnect with a new session.
I can then check and see if this has reconnected and created a 'clean' authentication entry in the logs.